1. Field of the Invention
The invention relates to circuits and logic circuits generally, and in particular to logic circuits having non-volatile reprogrammable operating characteristics.
2. Description of the Related Art
The need for extremely low power and high frequency device elements for digital signal processors, logic devices, and data storage and retrieval has led to a rapid increase in research on negative differential resistance (NDR) devices, particularly resonant tunneling diodes (RTDs), which offer high frequency operation (greater than 100 GHz) and extremely low operating voltages (less than 400 meV). The resonant tunneling diode has a unique N-shaped current-voltage (I-V) response characteristic (FIG. 1) that provides a negative differential resistance above the voltage corresponding to an initial peak current. Voltages that define the peak, negative differential resistance and valley regimes can be exploited to define "high" and "low" states of a logic or memory cell. Resonant tunneling diodes feature a ultra-high speed transient response, with a switching time of less than 1 picosecond, that allows operation in the terahertz frequency regime. When coupled with high speed transistors, resonant tunneling diodes form the basis for a highly functional, low component count architecture for logic, signal processing and memory applications. (See, for example, T. Whitaker, "Tunnel Diodes Break Through At Last" Compound Semiconductors 4, no. 3, (1998) pp. 36-41, P. Mazumder,et al, "Digital Circuit Applications of Resonant Tunneling Devices" Proc. IEEE 86, (1998) pp. 664-686, Niu, et al, "Circuit Modeling of Programmable Logic Gate Based on Controlled Quenching of Series-Connected Nigative Differential Resistance Devices", 1997 IEEE International Symposium on Circuits and Systems, pp 1628-1631, and Goser ,K. and Pacha, C. "System and Circuit Aspects of Nanoelectronics ESSCIRC '98, 24.sup.th European Solid-State Circuits Conference", The Hague, September 1998, all incorporated herein by reference.) FIG. 2 (reproduced from Whitaker) shows a static random access memory (SRAM) cell made up of two resonant tunneling diodes (RTDs) and a single heterojunction field effect transistor (HFET). A single bit is stored at the storage node, which is also the source of the HFET. FIG. 3 shows the measured load line for the cell under 0.45V bias. Dots show the stable latch states.
A fast and highly compact logic element known as a MOBILE (monostable-bistable transition logic element) can be fabricated from a FET and two RTDs, as shown in FIG. 4 (see, for example, P. Mazumder, supra, page 672 and K. J. Chen, et al, "Monostable-Bistable Transition Logic Elements (MOBILEs) Based on Monolithic Integration of Resonant Tunneling Diodes and FETs" Jpn. J. Appl. Phys. 34, Pt. 1, No. 2B, 1199 (1995), incorporated herein by reference).
Multiple-value logic circuits have been fabricated from parallel combinations of an RTD subsystem which consists of an RTD in series with a load resistor, as shown in FIGS. 5a (see, for example P. Mazumder, supra, page 672 and L. J. Micheel and M. J. Paulus, "Differential Multiple-Valued Logic Using Resonant Tunneling Diodes", Proc. 20th Intl. Symp. Multiple-valued Logic, Charlotte, N.C., 1990, pp. 189-195, incorporated herein by reference).
Typical memory cells and logic circuits using resonant tunneling diodes or other negative differential resistance devices are described in the following U.S. patents incorporated herein by reference: U.S. Pat. No. 5,128,894 to Lin; U.S. Pat. No. 5,162,877 to Mori; U.S. Pat. No. 5,265,044 to Singh; U.S. Pat. No. 5,229,623 to Tanoue, et al; U.S. Pat. No. 5,280,445 to Shieh et al; U.S. Pat. No. 5,294,566 to Mori; U.S. Pat. No. 5,311,465 to Mori; U.S. Pat. No. 5,390,145 to Nakasha, et al; U.S. Pat. No. 5,408,107 to Neikirk, et al; U.S. Pat. No. 5,477,169 to Shen et al; U.S. Pat. No. 5,535,156 to Levy et al; U.S. Pat. No. 5,646,884 to van der Wagt; U.S. Pat. No. 5,714,891 to Lin et al; U.S. Pat. No. 5,745,407 to Levy, et al; U.S. Pat. No. 5,789,940 to Taddiken; U.S. Pat. No. 5,773,996 to Takao; U.S. Pat. No. 5,811,832 to Alphenaar et al; U.S. Pat. No. 5,815,008 to Williamson III; U.S. Pat. No. 5,869,845 to van der Vagt, et al; U.S. Pat. No. 5,883,829 to van der Vagt; U.S. Pat. No. 5,903,170 to Kulkarni, et al; U.S. Pat. No. 5,930,323 to Tang; U.S. Pat. No. 5,953,249 to van der Vagt; and U.S. Pat. No. 5,981,969 to Yuan et al.
A disadvantage of typical memory and logic circuits based on resonant tunneling diodes is that, typically, the memory or logic state created in such circuits is volatile.
Another recent development in computer technology is the creation of sensors and non-volatile memory cells using magnetic devices based on giant magnetoresistance (GMR) or spin-dependent tunneling junctions (STJ). A typical giant magnetoresistance device or spin-dependent tunneling junction device consists of two or more ferromagnetic films separated by a non-magnetic spacer layer. The essential characteristic of such a device is that the electrical resistance of the structure depends upon the relative orientation of the magnetization of the individual magnetic layers: the resistance is low when the magnetic moments of the layers are parallel, and high when they are antiparallel. The relative orientation of the magnetization can be changed by applying a magnetic field. (This is typically accomplished by applying a fringe field from current pulse through an adjacent wire.) The values of the resistance of these devices and their change in resistance with the change in magnetic field depend upon the materials used and details of the structure. The change in resistance that can be brought about can range from a few percent to well over 100%. Such devices can be tailored to exhibit a resistance which can be reversibly and continuously varied between a high and low value with applied magnetic field, with a single stable resistance state at zero applied magnetic field. Such devices are currently used as sensors in magnetic disk drive read heads, for example. Alternatively, these magnetic devices can also be tailored to exhibit at least two stable resistance values, corresponding, for example, to parallel and antiparallel orientation of the magnetization of the magnetic layers--the change in orientation of the magnetization is typically reversible and non-volatile (a voltage is not required to maintain a specific orientation). Typical giant magnetoresistance devices and spin-dependent tunneling junction devices and the use of these devices in memory elements are described, for example, in the following publications and U.S. patents, incorporated herein by reference: Prinz, G. A., "Magnetoelectronics", Science, 282, pp 1660-1663, U.S. Pat. No. 5,287,238 to Baumgart, et al; U.S. Pat. No. 5,459,687 to Sakakima, et al; U.S. Pat. No. 5,477,482 to Prinz; U.S. Pat. No. 5,587,943 to Torok, et al; U.S. Pat. No. 5,629,922 to Moodera, et al; U.S. Pat. No. 5,640,343 to Gallagher, et al; U.S. Pat. No. 5,661,062 to Prinz; U.S. Pat. No. 5,732,016 to Chen, et al; U.S. Pat. No. 5,764,567 to Parkin; U.S. Pat. No. 5,793,697 to Schuerlein; U.S. Pat. No. 5,801,984 to Parkin; U.S. Pat. No. 5,835,314 to Moodera, et al; U.S. Pat. No. 5,841,692 to Gallagher, et al; U.S. Pat. No. 5,852,574 to Naji; U.S. Pat. No. 5,936,293 to Parkin; U.S. Pat. No. 5,936,882 to Dunn; U.S. Pat. No. 5,949,707 to Pohm, et al; U.S. Pat. No. 5,966,322 to Pohm et al; U.S. Pat. No. 5,969,978 to Prinz; U.S. Pat. No. 6,005,800 to Koch, et al; and U.S. Pat. No. 6,021,065 to Daughton, et al.